How to Create a Client Document Center on Subsites

I have a request from a client (tax preparation service) to create a client document center where registered users can share documents with the admin.

1. Each user should be able to access a list of files that they have uploaded and/or that have been uploaded by the admin for them.

2. Each user must be able to download any of the files that have been uploaded by or for them.

3. Each user must be able to delete files that have uploaded by or for them.

4. The functionality should (preferably) exist outside the post/page publishing functionality of WordPress. (E.g., it would be ideal if the user should be log in, upload their W2 forms and log out.)

5. If one party uploads a file, the other should get an email notification. The notification should include a download link (however, the recipient should be required to be logged in before downloading the file).

Does anyone know of any plugin out there that has similar functionality? If not, is there any interest from the community in having the good folks over here at WPMU DEV create one?

I’d welcome your feedback and suggestions.